1. Field of the Invention
The present invention relates to countermeasures against EMI (electromagnetic interference) in electronic devices, and in particular to a technique of using a quasi-grounding method to reduce radiant noise energy in an electronic device connected to a plurality of cables such as communication wires, power source lines, and interface lines.
2. Description of the Related Art
First, a quasi-grounding method will be described as a background of the invention.
Quasi-Grounding
As shown in FIG. 1, assuming that a hole is dug in the ground 11 and a wire 12 is buried therein, the wire 12 does not radiate any electromagnetic.
As shown in FIG. 2, a housing 10 enabling a radiant noise energy level to be reduced below the regulation value level of EMI, can be regarded as “quasi-ground”, and there is no radiated electromagnetic wave from the wire 12 fixed to the housing 10 by means of screw.
Referring to FIG. 3, a housing 10 accommodates a group of communication system packages 21 and a group of control system packages 22. Each of the communication system packages 21 is connected to a communication wire such as a twisted cable extending from a front plate (quasi-ground surface) 20 of the housing 10. The control system packages 22 each have no communication wires and include high-speed central processing units (CPU), memory packages, clock packages, (CLK), and interface packages (INT).
In FIG. 3, a quasi-ground surface 20 is a surface that reduces a radiant noise energy level below the regulation value level of EMI, and does not radiate any electromagnetic wave from the wire fixed thereto.
FIG. 4A is a sectional side view of a communication system package having a communication wire. In FIG. 4A, an earth pin 3E (also referred to as an earth terminal or “E pin”) of a communication LSI 3F, a plurality of LSIs (Large Scale Integration) implemented in the package, and an E (earth) pin of an IC component are connected to the housing 10 through an E (earth) 3D of a printed board (PWB), a connector 3C of a back board (BWB), an E (earth) layer 3B of the back board, and bosses 3A.
On the other hand, a power pin 3Q of the communication LSI 3F and a power pin of LSIs and IC components implemented in the package are connected to the contact point of one end of a decoupling inductance 3N and one end of a capacitor 3P. The other end of capacitor 3P is connected with E (earth) 3D of the printed board (PWB). The other end of inductance 3N is connected with a power layer 3M of the printed board via a connector 3L and a power layer 3K of the back board.
Also, the communication wires 3H and 3J, connected with communication output pins 8H and 8J of the communication LSI 3F through a common mode choke 3G, are taken outwards from the housing 10 via an hole opened on the quasi-ground surface 20 of the front side of the housing 10.
Here, the E (earth) 3D of the printed board and the power 3M of the printed board are respectively an E (earth) layer and a power layer on the printed board of the package having the same communication wire connected thereto.
Radiant Noises
By the way, there are two kinds of noise energies radiated from the communication wires 3H and 3J: one is a normal mode noise current having a noise current with a same level that goes between communication wires; and the other is a common mode noise current flowing in the same direction through the communication wires 3H and 3J.
The normal mode noise current is a noise energy generated by an individual communication LSI of each communication wire package. On the other hand, the common mode noise energy is an energy obtained by combining all of earth noises generated by the control system packages 22 and the communication system packages 21, which are accommodates in the housing 10 of FIG. 3. The common mode noise energy is much larger than that of the normal one, and therefore it is the biggest factor of radiation as if the communication wire is a monopole antenna.
As will be described later, the present invention provides a method and device allowing a reduction of the common mode noise to mainly induce radiant noises. The common mode noise will be described below in detail.
Common Mode Noise
FIG. 5 shows an outline of impedance factors causing a great common mode noise between the quasi-ground 20 and the E (earth) pin 3E of the communication LSI 3F.
Referring to FIG. 5, the presence of impedance (housing impedance) of a metallic member composing the housing IC is a big factor that the common mode noise is generated between the housing 10 and the quasi-ground surface 20 with the smallest noise energy.
The E (earth) current flows through the impedance (boss-E impedance) of a plurality of metallic bosses 3A from the E (earth) layer 3B of the back board (BWB), and thereby a common mode noise is generated at this boss-E impedance.
The E (earth) current from all packages of the control system packages 22 and the communication system packages 21 flows through the impedance (BWB-E impedance) of the E (earth) layer 3B of the back board, and thereby a common mode noise is caused.
The current branching out from the E (earth) 3D of the printed board of the communication system packages flows through the impedance (connector-E impedance) of a plurality of E (earth) pins of the connector 3C, and thereby a common mode noise is caused.
The current from many LSI or IC components implemented to this package flows through the impedance (PWB-E impedance) of the E (earth) 3D provided on the printed board (PWB) of a package having a communication wire, and thereby a common mode noise is caused.
As shown in FIG. 6A, therefore, when the E (earth) currents flow through many kinds of above-described impedances existing between the quasi-ground surface 20 and the E (earth) pin of communication LSI 3F, which is indicated by a common impedance 40 in FIG. 5, a large amount of common mode noise energy as denoted by a reference numeral 51 of FIG. 6B exists in the E (earth) pin 3E of communication LSI 3F implemented to the printed board of the package having communication wire. Since the communication. LSI 3F functions with respect to this E pin 3E, the large amount of common mode noise 51 (see FIG. 6) generated in the common impedance 40 is radiated as it is towards the communication wires 3H, 3J connected with the communication output pins 8H, 8J of the communication LSI 3F.
FIGS. 6(a) and 6(b) each show a relationship between the common impedance 40 and the quasi-ground 20, wherein FIG. 6(a) schematically shows components forming the common impedance 40.
Since the internal E (earth) of communication LSI 3F as shown in FIG. 6(b) works with respect to the E pin 3E, the common mode noise on the internal E (earth) of communication LSI 3F, that is, the common mode noise 51 on E pin 3E of communication LSI 3F is radiated as it is to the communication wire output pins 8H, 8J of communication LSI 3F. Therefore, the internal E (earth) of communication LSI 3F and the E (earth) pin 3E can be recognized as a common E (earth) 50 as shown in FIG. 6(c) and it can be also regarded that the communication wire output pins 8H, 8J of communication LSI 3F are also connected to the common E (earth) 50.
As shown in FIG. 6(c), if the communication LSI 3F is omitted, an equivalent configuration in which the common mode choke 3G inputs the same common mode noise 51 is obtained. As the result, the communication wires 3H, 3J play the role of a monopole antenna connected to the quasi-ground surface 20 to radiate the noise energy exceeding the regulation value.
As described before, this radiant noise energy means the direct radiation of common mode noise energy 51 caused by the E (earth) current flowing through the common mode impedance 40.
The description of electric field intensity, which is selected because of easy theoretical analysis although electromagnetic wave is composed of electric field and magnetic field, is shown in FIG. 7.
An electric field strength E (V/m) can be obtained by the following formula:E=12.6×10−7f·h·i/d(V/m)∝f·h·i/d(V/m)  (1),where i: a current value (A), h: antenna height (m), and f: frequency (Hz), and d: distance (m) away from the monopole antenna.
In order to make this electric field intensity E smaller, current i, antenna height h, and frequency f are made smaller, and/or distance d greater.
In order to equivalently set the current i longitudinally flowing through he antenna to zero, a pair of quasi-grounding capacitors are employed in a conventional communication system package as shown in FIG. 8.
Conventional Circuit Configuration
There has been proposed a quasi-grounding method and device in Japanese Patent Application Unexamined Publication No. 11-30798, which was filed by Suzuki (the present inventor) et al. The details thereof will be described hereafter.
As shown in FIG. 8, a capacitor 60 having a property of high frequency is connected between the quasi-ground surface 20 and a point 6H connecting one coil of common mode choke 3G with the communication wire 3H, and a capacitor 61 having a property of high frequency is connected between the quasi-ground surface 20 and a point 6J connecting the other coil of common mode choke 3G with the communication wire 3J. The capacitors 60, 61 are hereinafter called quasi-grounding capacitors because the respective capacitors 60 an 61 make short circuits in a high frequency range between the communication wires 3H, 3J and the housing. As the result, the radiant noise energy level of the communication wires 3H and 3J are substantially equal to that of the quasi-ground (housing) 20.
In other words, the quasi-grounding capacitors 60, 61 cause the radiant noise energy to flow into the quasi-ground surface 20 to short-circuit the common impedance in high frequencies, preventing common mode noise energy 51 from being generated. The example shown in FIG. 8 is composed to combine the common mode choke 3G with the quasi-grounding capacitors 60, 61 so that a large amount of common mode noise energy is eliminated effectively.
FIG. 9A shows a sectional view of a conventional package having the circuit configuration as shown in FIG. 8. Compared with the circuit configuration of FIG. 4A, the quasi-grounding capacitors 60, 61 are additionally connected between the communication wires 3H, 3J and the quasi-ground surface 20.
Since the radiant noise energy of communication wires 3H, 3J flows into the quasi-ground surface 20 through the quasi-grounding capacitors 60, 61, with the increase of number of communication system packages 21 accommodated in the housing 10, the radiant noise energy level 30 of quasi-ground surface 20 is also increased (see FIG. 9B). In this case, even if all of the communication system packages 21 are accommodated in the housing, it is necessary to keep the radiated noise energy level 30 of the quasi-ground surface 20 within the regulation value 70. For this purpose, various countermeasures against noise energy (EMI) have been employed for each internal circuit accommodated in he housing, such as back board, control system packages, and communication system packages.
According to the conventional method described referring to FIGS. 8 and 9A, 9B, the quasi-grounding can be effectively realized in the case where a common mode radiant noise energy of 30 MHz or more is reduced from analog communication wires for a voice frequency band of 4 KHz or less. The reason is that a frequency difference between the voice signal on the communication wires and the noise energy to be eliminated is large. Therefore, after the common mode radiant noise energy is removed by the common mode choke 3G and the quasi-grounding capacitors 60, 61, the normal mode signal level is not substantially reduced.
Here, the frequency of 30 MHz is adopted because the radiant noise of 30 MHz or more is regulated by the International Special Committee on Radio Interference (CISPR) international standards, the Voluntary Control Council for Interference by Information Technology Equipment (VCCI) domestic standards, the FCC federal standards, and the EN55022 European standards.
However, with the increase of signal speed on digital communication wires, for example, a gigabit Ethernet with a transmission rate of Giga bps order in recent years, the frequency difference between the signal and the common mode noise is getting smaller. Accordingly, the quasi-grounding capacitors employed in the prior art cause the normal mode signal level on the communication wires to be also attenuated when the frequency of a signal on communication wires is increased to near that of the common mode noise.
Now it is required to develop such a technology that can achieve an economical countermeasure against EMI under high-speed transmission on communication wires with reliability and stability.